The French concern is set to offer $4.5 billion, nearly equal to the $4.7 billion offer made by MidAmerican Energy Holdings Co. for the entire company. Shareholders are scheduled to vote on the MidAmerican offer on Dec. 23, The Baltimore Sun reported Wednesday.

EDF's offer values Constellation at $52 a share, while the MidAmerican offer values the company at $26.50 a share bid, sources told the Sun.

"It definitely clouds the picture and gives the shareholders something to contemplate," said analyst Paul Justice said.

MidAmerican's bid trumped EDF's previous $35 per share bid for the entire company, as two U.S. private equity firms said the MidAmerican offer was more dependable and would be more likely to pass regulatory hurdles.

"We are focused on completing our merger with MidAmerican Energy Holdings and, beyond that, we cannot comment on market rumor and/or speculation," Constellation spokesman Rob Gould said Tuesday.